Progressing cavity motors, also known as Moineau-type motors (after the inventor of U.S. Pat. No. 1,892,217), including stator devices used therein, have been used in drilling applications for many years. See, for example, the following U.S. Pat. Nos. which are incorporated herein by reference: U.S. Pat. Nos. 3,840,080; 3,912,426; 4,415,316; 4,636,151; 5,090,497; 5,171,138; 5,417,281; 5,759,109; and 6,183,226.
Conventional Moineau pump and motor art has used rubber or elastomer materials bonded to steel for the stator contact surface. Such elastomers include not only natural rubber, but also synthetics, such as G.R.S., Neoprine, Butyl and Nitrile rubbers and other types such as soft PVC. For example, U.S. Pat. No. 5,912,303, incorporated herein by reference, discloses a polyene terpolymer rubber composition that is vulcanized for applications in the automotive industry. EPDM, a terpolymer, is highly resistant to weather, ozone and heat aging but is not oil resistant. The '303 patent teaches blending nitrile rubber (NBR), which is oil resistant, with EPDM to obtain the advantages of both NBR and EPDM. The rubber is vulcanized and then used in tires, hoses, windshield wipers and the like that are subjected to weather and the like.
Rubber stators in down hole drilling motors are subjected to a harsh environment involving both higher temperatures, hydrocarbon immersion and dynamic loading. The key here in down hole motors has been to make the elastomer property soft enough for injection molding and soft enough to maintain the sealed cavity, yet be hard enough to be able to withstand the abrasive wear from the working contact between the rotor and the stator. U.S. Pat. No. 5,620,313, entitled “Worm Pump For Flowable Media,” utilizes a stator wall composed of a rubber with a Shore A hardness of 90 to 95 (tested in accordance with ASTM D2240). Such a hard elastomer property is desirable for withstanding the abrasive wear found in conventional down hole drilling motors. However, such a hard material is difficult to injection mold, resulting in expensive manufacturing costs. Thus, the prior art has not been able to achieve a satisfactory balance for use in down hole motors, regarding durability in operation but easier to manufacture.
Additionally, drilling applications generally involve high-temperature environments. U.S. Pat. No. 6,183,226 teaches that rubber used as the stator contact surface is not desirable in high-temperature environments because of its low heat conductivity. U.S. Pat. Nos. 6,183,226 and 5,417,281 disclose use of composites formed from fiberglass, resin, and elastomer. Further, as progressive cavity devices increase in diameter or length or both (as in oil and gas drilling applications), flow characteristics to maintain a successful and long-lasting bond of the rubber to steel housing becomes quite difficult. Moreover, where hydrocarbons make up the material to be pumped, such as in oil and diesel-based drilling mud used in some drilling operations, some rubber compounds are known to deteriorate.